Workpiece holding apparatuses have been used for a number of years to hold solid and annular workpieces for subsequent machining or grinding. As is known, workpiece holding apparatuses may be mechanically actuated or hydraulically actuated. Mechanically actuated workpiece holders typically have a collet that is adapted for holding a workpiece by collapsing around the workpiece or by expanding within the workpiece. The collet is generally cylindrical and includes a number of leaves that are defined by relief slots that allow the collet to radially expand and contract. The collet also has a tapered portion that cooperates with a corresponding tapered portion of an actuating mechanism that actuates the collet. The actuating mechanism is linearly displaceable to impose a linear force on the collet, wherein the linear displacement of the actuating mechanism is converted to radial displacement of the collet via the corresponding tapered portions.
Unfortunately, however, workpiece holding apparatuses having collets tend to have some drawbacks. Mechanically actuated workpiece holders using conventional collets often yield line-contact engagement with a workpiece wherein only one or two leaves of the collet fully engage the workpiece. This line-contact engagement may yield insufficient holding force on the workpiece wherein the workpiece may rotate relative to the collet. Moreover, line-contact engagement may render the collet unfit for conforming to an out-of-round workpiece. Worse yet, line-contact engagement may also induce lobing of the workpiece during the machining process, thereby creating an out-of round workpiece. Such out-of-round workpieces must then be subjected to a subsequent finishing process to smooth out-of-round surfaces on the workpiece into a more suitably rounded cylindrical surface to establish an acceptable final product. Such subsequent finishing processes are time consuming, expensive and relatively inefficient.
In contrast to mechanically actuated workpiece holders, hydrostatic or hydraulically actuated workpiece holders typically include a body, a cylindrical expansion sleeve mounted around the body and defining a fluid chamber therebetween, seals between the expansion sleeve and the body for sealing off the fluid chamber, and one or more workpiece sleeves mounted around the expansion sleeve. A hydraulic system communicates with the fluid chamber by hydraulic fluid circuits and pressurizes the fluid chamber to radially expand the expansion sleeve and, in turn, radially displace the workpiece sleeves into engagement with a workpiece. The performance of typical hydraulically actuated workpiece holders may be constrained in some applications, such as where workpiece tolerances are wide. In such extraordinary applications, a typical hydraulically actuated workpiece holder may not exhibit radial displacement required to grip all workpieces falling within the wide tolerance band.